On Tue, 8 Jul 2025 at 10:54, Luc Grosheintz <luc.groshei...@gmail.com> wrote:
>
> This implements P3029R1. In P3029R1, the CTAD for span is refined to
> permit deducing the extent of the span from an integral constant, e.g.
>
> span((T*) ptr, integral_constant<size_t, 5>{});
>
> is deduced as span<T, 5>. Similarly, in
>
> auto exts = extents(integral_constant<int, 2>);
> auto md = mdspan((T*) ptr, integral_constant<int, 2>);
>
> exts and md have types extents<size_t, 2> and mdspan<double,
> extents<size_t, 2>>, respectively.
>
> PR libstdc++/120914
>
> libstdc++-v3/ChangeLog:
>
> * include/std/span (span): Update CTAD to enable
> integral constants [P3029R1].
> * include/std/mdspan (extents): ditto.
> (mdspan): ditto.
> * testsuite/23_containers/span/deduction.cc: Test deduction
> guide.
> * testsuite/23_containers/mdspan/extents/misc.cc: ditto.
> * testsuite/23_containers/mdspan/mdspan.cc: ditto.
>
> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> ---
> libstdc++-v3/include/std/mdspan | 8 ++----
> libstdc++-v3/include/std/span | 20 ++++++++++++++-
> .../23_containers/mdspan/extents/misc.cc | 20 +++++++++++++++
> .../testsuite/23_containers/mdspan/mdspan.cc | 25 +++++++++++++++++++
> .../testsuite/23_containers/span/deduction.cc | 3 +++
> 5 files changed, 69 insertions(+), 7 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
> index 0fd78570b3e..152dcb3e92a 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -406,10 +406,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename _IndexType, size_t... _Counts>
> auto __build_dextents_type(integer_sequence<size_t, _Counts...>)
> -> extents<_IndexType, ((void) _Counts, dynamic_extent)...>;
> -
> - template<typename _Tp>
> - consteval size_t
> - __dynamic_extent() { return dynamic_extent; }
> }
>
> template<typename _IndexType, size_t _Rank>
> @@ -419,7 +415,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename... _Integrals>
> requires (is_convertible_v<_Integrals, size_t> && ...)
> explicit extents(_Integrals...) ->
> - extents<size_t, __mdspan::__dynamic_extent<_Integrals>()...>;
> + extents<size_t, __detail::__maybe_static_ext<_Integrals>...>;
>
> struct layout_left
> {
> @@ -1316,7 +1312,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> && (sizeof...(_Integrals) > 0)
> explicit mdspan(_ElementType*, _Integrals...)
> -> mdspan<_ElementType,
> - extents<size_t, __mdspan::__dynamic_extent<_Integrals>()...>>;
> + extents<size_t, __detail::__maybe_static_ext<_Integrals>...>>;
>
> template<typename _ElementType, typename _OIndexType, size_t _Nm>
> mdspan(_ElementType*, span<_OIndexType, _Nm>)
> diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
> index 49ab9109d83..5629a71b9bd 100644
> --- a/libstdc++-v3/include/std/span
> +++ b/libstdc++-v3/include/std/span
> @@ -476,6 +476,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> };
>
> // deduction guides
> + namespace __detail
> + {
> + template<typename _Tp>
> + concept __integral_constant_like = is_integral_v<decltype(_Tp::value)>
> + && !is_same_v<bool, remove_const_t<decltype(_Tp::value)>>
> + && convertible_to<_Tp, decltype(_Tp::value)>
> + && equality_comparable_with<_Tp, decltype(_Tp::value)>
> + && bool_constant<_Tp() == _Tp::value>::value
> + && bool_constant<static_cast<decltype(_Tp::value)>(_Tp()) ==
> _Tp::value>
> + ::value;
> +
> + template<typename _Tp>
> + constexpr size_t __maybe_static_ext = dynamic_extent;
> +
> + template<__integral_constant_like _Tp>
> + constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
Are the braces here to detect narrowing conversions?
(The paper doesn't mention why they're used, as far as I can see)
> + }
>
> template<typename _Type, size_t _ArrayExtent>
> span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
> @@ -489,7 +506,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> template<contiguous_iterator _Iter, typename _End>
> span(_Iter, _End)
> - -> span<remove_reference_t<iter_reference_t<_Iter>>>;
> + -> span<remove_reference_t<iter_reference_t<_Iter>>,
> + __detail::__maybe_static_ext<_End>>;
>
> template<ranges::contiguous_range _Range>
> span(_Range &&)
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> index e71fdc54230..bca8901685d 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> @@ -97,6 +97,25 @@ test_deduction(Extents... exts)
> VERIFY(e == expected);
> }
>
> +constexpr bool
> +test_integral_constant_deduction()
> +{
> + auto verify = [](auto actual, auto expected)
> + {
> + static_assert(std::same_as<decltype(actual), decltype(expected)>);
> + VERIFY(actual == expected);
> + };
> +
> + constexpr auto c1 = std::integral_constant<size_t, 1>{};
> + constexpr auto c2 = std::integral_constant<int, 2>{};
> +
> + verify(std::extents(1), std::extents<size_t, dyn>{1});
> + verify(std::extents(c1), std::extents<size_t, 1>{});
> + verify(std::extents(c2), std::extents<size_t, 2>{});
> + verify(std::extents(c1, 2), std::extents<size_t, 1, dyn>{2});
> + return true;
> +}
> +
> constexpr bool
> test_deduction_all()
> {
> @@ -104,6 +123,7 @@ test_deduction_all()
> test_deduction<1>(1);
> test_deduction<2>(1.0, 2.0f);
> test_deduction<3>(int(1), short(2), size_t(3));
> + test_integral_constant_deduction();
> return true;
> }
>
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> index 9252273bf66..a650fb19bdf 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> @@ -245,6 +245,28 @@ test_from_pointer_and_shape()
> return true;
> }
>
> +constexpr bool
> +test_from_pointer_and_integral_constant()
> +{
> + std::array<double, 6> buffer{};
> + double * ptr = buffer.data();
> +
> + auto verify = [ptr](auto actual, auto exts)
> + {
> + auto expected = std::mdspan<double, decltype(exts)>(ptr, exts);
> + static_assert(std::same_as<decltype(actual), decltype(expected)>);
> + VERIFY(actual.extents() == expected.extents());
> + };
> +
> + auto c3 = std::integral_constant<int, 3>{};
> + auto c6 = std::integral_constant<int, 6>{};
> +
> + verify(std::mdspan(ptr, 6), std::extents(6));
> + verify(std::mdspan(ptr, c6), std::extents(c6));
> + verify(std::mdspan(ptr, 2, c3), std::extents(2, c3));
> + return true;
> +}
> +
> constexpr bool
> test_from_extents()
> {
> @@ -616,6 +638,9 @@ main()
> test_from_pointer_and_shape();
> static_assert(test_from_pointer_and_shape());
>
> + test_from_pointer_and_integral_constant();
> + static_assert(test_from_pointer_and_integral_constant());
> +
> test_from_extents();
> static_assert(test_from_extents());
>
> diff --git a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> index dce6cedf89b..c66db90222e 100644
> --- a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> +++ b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> @@ -80,4 +80,7 @@ test01()
>
> std::span s12(const_cast<const std::span<int>&>(s5));
> static_assert( is_dynamic_span<int>(s12) );
> +
> + std::span s13(a.data(), std::integral_constant<size_t, 3>{});
> + static_assert( is_static_span<long, 3>(s13));
> }
> --
> 2.49.0
>
